1. Field of Invention
This invention relates to ferroelectric liquid crystal spatial light modulation and specifically to a method and apparatus for addressing pixels in a ferroelectric liquid crystal array.
2. Prior Art
1. S. T. Lagerwall, J. Wahl and N. A. Clark, 1985 International Display Research Conference-Ferroelectric Liquid Crystals for Displays, 1985 IEEE.
2. U.S. Pat. No. 4,367,924 to Clark et al entitled "Chiral Smectic C or H Liquid Crystal Electro-Optical Device".
3. U.S. Pat. No. 4,563,059 to Clark et al entitled "Surface Stabilized Ferroelectric Liquid Crystal Devices".
A ferroelectric liquid crystal spatial light modulator is a bistable device which can be switched between two stable states representing two different orientations of the optical axis of the ferroelectric liquid crystal confined between two spaced apart transparent windows or electrodes. Control of the orientation of the optical axis is affected by applying a strong electric field across the liquid crystal volume perpendicular to the electrodes. A positive electric field created by a positive voltage pulse selects one orientation of the optical axis while a negative electric field created by a negative voltage pulse will select the other orientation.
The bistable nature of the ferroelectric liquid crystal lends itself to the formation of an array device formed of columns and rows of spaced apart electrodes with a switchable cell, which makes up a pixel, of ferroelectric liquid crystals formed at each intersection of the columns and rows. The voltage pulse applied to each pixel to switch the state of the pixel is called a write pulse. The threshold characteristics of the liquid crystal determines the magnitude and duration of the voltage pulse that must be applied to switch the state of the optical axis of each pixel.
To protect against detrimental electrolytic effects which occur as a consequence of having a long term DC component applied to the device, it is necessary to apply a series of under threshold opposite polarity voltage pulses to the device before or after or both before and after the write pulse. This has the effect of zeroing the average DC voltage component across the cell and thereby avoids the detrimental electrolytic effects. The price paid for this is that the total time needed to write is about 4-5 times the basic write time. That is, the device can be operated at only about 1/4th to 1/5th of the speed that would be indicated by the optical rise time. It is therefore of great importance to carefully design the write and the DC zeroing pulse sequences.
Lagerwall et al, ref 1, describe a scheme for writing a column of pixels, however, the pulse sequences they suggest are complex and can write only one type of pixel (+1 or -1) with each column access. Thus, two column-write cycles are needed to write a spatial sequence or column of +1 or -1 states. In addition, their scheme requires that any pixel that is already in its desired state, must be toggled into the opposite state and then toggled back into its original state during the write cycle. All of these are undesirable properties. A scheme for writing a column of pixels that does not have these undesirable properties is needed.